The geochemical controls on anaerobic microbial ecology in a phenol-contaminated sandstone aquifer.

The Permo-Triassic sandstone aquifer is one of the most important drinking water resources in the UK. Beneath the Four Ashes site, the aquifer is contaminated with toxic levels of phenolic compounds and inorganic species. Although the hydrochemistry of the groundwater has been fully characterised, no research has assessed contaminant plume development with time using full groundwater profiles. Also, studies have investigated the aerobic microbial ecology at the site, but no research has assessed the anaerobic communities. Anaerobes are important for the remediation of the contamination, as the majority of the contaminated groundwater is anoxic. Furthermore, research has demonstrated that the structure of planktonic (free-living) eubacterial communities at 30m below ground level (mbgl) differ significantly from the community attached to quartz sand suspended in the aquifer at the same depth. However, the diverse mineralogical matrices in sandstones may influence microbial attachment, and therefore the communities attached to quartz may not be representative of the true indigenous attached communities. The aim of this thesis was to update the knowledge on the groundwater hydrochemistry, and to investigate the effects of contaminant load and substratum properties on microbial attachment and anaerobic microbial ecology. Groundwater samples were taken from multiple depths using 2 depth-discrete boreholes. Concentrations of phenolic compounds in the groundwater were measured using high performance liquid chromatography, and inorganic species were measured by ion chromatography. Chemical profiles show that the groundwater hydrochemistry at borehole 59 (130m from source) remained stable between 1998 and July 2009. However, the data reveals that the distribution of the contaminants changed in August 2009, due to the onset of the pump and treat remediation strategy implemented at the site. Data from borehole 60 (350m from source) shows that the concentration of total phenolics increased markedly at 42- 45mbgl between 1998 and 2009, due to variations in contaminant spillage on site, potentially resulting in the plume extending further than the deepest sampling point (45mbgl). Denaturing gradient gel electrophoresis (DGGE) of amplified 165 rRNA, napA (nitrate-reductase) and dsrB (dissimilatory sulphate reductase) reveal that the structure of eubacterial and anaerobic communities varied with depth down the groundwater profile, and were therefore influenced by contaminant load. The DGGE profiles of planktonic communities were compared to the communities attached to 3 different geological substrata suspended in the aquifer under the same hydrochemical conditions (i.e. same depth). Profiles reveal that the planktonic communities were highly dissimilar to the attachment communities on all surfaces, and at all samplings. Furthermore, DGGE profiles of the attached communities were compared. Results reveal that the communities attached to 5hap granite and sandstone cluster separately to the community attached to quartz, suggesting that substratum properties are an important control governing microbial attachment. This was investigated by quantifying microbial attachment to 6 different geological coupons (apatite, 5hap granite, hematite, orthoclase, quartz and shale) suspended in the aquifer. Each coupon provided different physiochemical conditions (hydrophobicity, surface charge and nutrient chemistry) to the system. Results indicate that the most extensive attachment was on coupons that contained at least one microbial growth nutrient (Ca, P, K, Na, Fe, Mn); hematite, apatite, orthoclase and particularly 5hap granite, which contains the most nutrients. The least attachment was witnessed on the quartz surface, which contains no microbial nutrients. Quartz has similar surface charge and roughness to orthoclase, and a similar hydrophobicity to the 5hap granite. The only difference between the surfaces is nutrient availability. This confirms that substratum chemistry, in particular nutrient content, is a major control on microbial attachment.